I'm using the latest release and it seems that CBTH isn't working on friendly nameplates within instances. It only works with the centered screen feature only.

Is this intended or a bug?

It would be awesome, if the healing numbers work the same way as damage just on friendly nameplates as healer.

This isn't possible due to API restrictions put in by Blizzard. Specifically attaching anything like text to a nameplate is considered a modification of the nameplate and modifying friendly nameplates in instances isn't allowed.